Extrusion & Post
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Melt Fracture in PU Extrusion
Problem Phenomenon
PU shows severe melt fracture at excessive extrusion rate, extrudate spirally twisted
Cause Analysis
PU melt elasticity too high, elastic recovery at the die exit exceeding the critical value; die L/D ratio too short <10, melt not fully relaxed; extrusion temperature too low
Solution
1) Reduce extrusion rate to 2-3m/min; 2) Increase die L/D ratio to 15-20; 3) Increase extrusion temperature to 60-120°C; 4) Add a small amount of low-elasticity resin to reduce system elasticity; 5) Gradual contraction die entrance design
Key Takeaway
Melt fracture is caused by elastic recovery exceeding the critical value; reducing speed and lengthening the die are the core countermeasures
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